On 08/28/23 19:33, Muchun Song wrote: > > > > On Aug 25, 2023, at 19:18, Usama Arif <usama.arif@xxxxxxxxxxxxx> wrote: > > > > The new boot flow when it comes to initialization of gigantic pages > > is as follows: > > - At boot time, for a gigantic page during __alloc_bootmem_hugepage, > > the region after the first struct page is marked as noinit. > > - This results in only the first struct page to be > > initialized in reserve_bootmem_region. As the tail struct pages are > > not initialized at this point, there can be a significant saving > > in boot time if HVO succeeds later on. > > - Later on in the boot, HVO is attempted. If its successful, only the first > > HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages > > after the head struct page are initialized. If it is not successful, > > then all of the tail struct pages are initialized. > > > > Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx> > > This edition is simpler than before ever, thanks for your work. > > There is premise that other subsystems do not access vmemmap pages > before the initialization of vmemmap pages associated withe HugeTLB > pages allocated from bootmem for your optimization. However, IIUC, the > compacting path could access arbitrary struct page when memory fails > to be allocated via buddy allocator. So we should make sure that > those struct pages are not referenced in this routine. And I know > if CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, it will encounter > the same issue, but I don't find any code to prevent this from > happening. I need more time to confirm this, if someone already knows, > please let me know, thanks. So I think HugeTLB should adopt the similar > way to prevent this. In this patch, the call to hugetlb_vmemmap_optimize() is moved BEFORE __prep_new_hugetlb_folio or prep_new_hugetlb_folio in all code paths. The prep_new_hugetlb_folio routine(s) are what set the destructor (soon to be a flag) that identifies the set of pages as a hugetlb page. So, there is now a window where a set of pages not identified as hugetlb will not have vmemmap pages. Recently, I closed the same window in the hugetlb freeing code paths with commit 32c877191e02 'hugetlb: do not clear hugetlb dtor until allocating'. This patch needs to be reworked so that this window is not opened in the allocation paths. -- Mike Kravetz
